1. Field of the Invention
The present invention relates to the field of data storage devices, and in particular, to magnetic disk storage systems and methods that compensate, on an ongoing and real-time basis, for variations in data storage media spin speeds to improve the capacity of storage media and to control data density on the storage media.
2. Relevant Background.
The use of magnetic disk storage systems for electronically storing data continues to grow worldwide. Along with this increased use, the information industries are demanding higher capacity systems with minimal wasted storage space that also provide high accuracy data storage and retrieval.
In a typical magnetic disk storage system, a user""s data is stored in a disk drive device that includes one or more storage media, such as disks, with storage or platter surfaces on each side. On each platter surface of the disk, the user""s data is stored, and later retrieved, with a read/write head that is positioned above the platter surface by a connected arm that has its radial movement controlled by an actuator and servo controller. During operation, the disk is rotated on a central spindle connected to a spin motor at a spin speed to move the platter surface relative to the read/write head. The data is typically stored in numerous concentric, circular tracks which are divided into sectors representing the smallest addressable block of data on the platter surface that is read or written.
In addition to the user""s data, each track contains information that describes the radial and rotational position of the read/write head relative to the disk and, also, provides location or reference information to the magnetic disk storage system to allow it to know the position of the user data. This information is generally placed in fields, i.e., servo position fields, positioned at regular intervals between the data sectors in each track. This enables the servo controller (or other positioning system) to periodically access the positional information and, in response, to move the arm and/or change spin speed of the disk as needed to accurately position the read/write head. The size of the interval between adjacent servo position fields is established during initial configuration of the disk based on a feedback rate needed to control spin speed and is typically not modified by the user.
Variation in spin speeds within disk drive devices is an ongoing operational problem associated with magnetic disk storage systems that must be addressed by system designers when placing the data sectors on each disk to ensure accurate data storage and retrieval. Each track on a disk includes servo position fields and data sectors that are separated by gaps or spaces that are not available for storing user""s data. Further, due to design considerations such as optimizing linear bit density, optimizing the length of position feedback intervals, and the like, it is sometimes necessary for servo position fields to be positioned within a sector, thereby splitting the sector into two fragments creating gaps between each fragment and the servo position field.
Ideally, it would be desirable to minimize the size of these gaps to provide more data storage space, but unfortunately spin speed variation negatively affects sector placement and the size of included gaps in magnetic disk storage systems. In this regard, the designer is forced to provide nominal or minimal gaps that are large enough to avoid overlap of data when data in a first sector is written on the disk at a maximum or high spin speed and in a second, adjacent sector at a minimum or slow spin speed. For example, a track on a disk may have two servo position fields positioned with 90 microseconds of data recording time between them. If the magnetic disk storage system experiences a spin speed variation of plus or minus 0.3 percent, only 89.73 microseconds of data storage is available when the disk is spinning 0.3 percent too fast but 90.27 microseconds is available when the disk is spinning 0.3 percent too slow. If a designer desires, for example, to record a 50 microsecond first sector followed by a second sector in this interval, the designer must assume that the disk is spinning at the maximum spin speed. As a result, this 50 microsecond length of data may require 55.72 percent (i.e., 50 microseconds/89.73 microseconds) of the available space. Consequently, writing of the second sector cannot begin until at least 55.72 percent of the distance between the servo positioning fields has past, even when the disk is spinning at the minimal spin speed. This forces the designer to provide a space of 50.3 microseconds for 50 microseconds of data in the first sector which results in a 0.3 microsecond gap or waste of storage space when the spin speed is design or nominal. A similar xe2x80x9cworst casexe2x80x9d speed variation determination is made for the placement of each sector. This results in relatively large gaps between sectors and servo positioning fields that reduce available storage space.
In addition to the gaps between sectors and between servo position fields and sectors, the size of the phase locked oscillator (PLO) field included in each sector is typically increased to account for spin speed variation. The PLO field is not useful for user data storage but rather contains timing information that is extracted by the disk drive device. The timing information in the PLO field is coupled with a timing recovery device included in the magnetic disk storage system to synchronize the incoming data stream with data in the sector and to, thereby, compensate for spin speed variations that may have affected the position of the data during writing and reading operations. Disk drive devices are generally designed such that during a read operation, a read or read gate signal is asserted when the read/write head is positioned over a PLO field. If the read gate signal is asserted too soon, the timing recovery device may receive noise or transients that interfere with the synchronization operation. If the read gate signal is asserted too late, an insufficient number of PLO bytes are read and synchronization fails.
As a result, the size of the PLO fields and the selection of a read gate assertion point are chosen with careful attention to the minimum information requirements of the phase locked oscillator but also based on the uncertainty of the data position caused by spin speed variation. Similar to the gap size design considerations, the designer must assume that data in each sector was written at maximum spin speed placing the data as late as possible on the disk and, further, that the data may be read back at minimum spin speed with the read gate asserted as early as possible in the sector. In this case, the assertion point for the read gate must be chosen such that it is always asserted over the PLO field. Additionally, the designer must select the size of the PLO field such that enough bytes of timing information are read in the case when the data was written at minimum spin speed placing the sector as early as possible on the disk and then read back at maximum spin speed asserting the read gate signal as late as possible. As can be appreciated, the designer typically increases the size of the PLO field to insure proper data storage and retrieval. Unfortunately, this further reduces the amount of space on the disk available for data storage.
Consequently, there remains a need for a method or system for compensating for spin speed variation in the disk drive device of magnetic disk storage systems that more effectively utilizes the available data storage space on data storage media while providing high accuracy data storage and retrieval.
To address the spin speed variation problems of prior magnetic disk storage systems, the present invention uses each servo position field""s actual detected location and measures the spin speed between servo position fields to synchronize the reading and writing of user data to the rotational position of the data storage media. This technique of using ongoing and real-time measurements of actual spin speeds, measuring the variations from a nominal or design spin speed, and applying a correction factor prior to the next data transfer operation enables the designer to optimize the storage space usage by employing smaller gaps and PLO fields.
According to one aspect of the invention, the present invention involves a method for use in a magnetic disk storage system for compensating for variation in spin speed of a data storage media, such as a disk, from a nominal or design spin speed. The method generally includes first providing the nominal spin speed for the data storage media in the particular magnetic disk storage system, with the nominal spin speed being stored in memory. The method continues with the measuring of the actual spin speed of the data storage media. In one preferred embodiment, this measurement is completed by measuring the time from one servo position field to the next sequential servo position field in a track on the data storage media. Next, the spin speed variation or error is determined by comparing the measured spin speed with the nominal spin speed. The method continues with applying a correction factor calculated based on the spin speed error to a data transfer parameter, such as channel reference frequency or gate assertion timing, prior to a subsequent data transfer process (i.e., a read or a write operation). In some embodiments, the method further includes verifying that the measured spin speed is within a range of spin speeds expected based on maximum spin speed change rates of the magnetic disk storage system, and if outside this range, the data transfer operation, i.e., a write operation, is blocked to prevent damaging data existing on the data storage media.
According to another aspect of the invention, a magnetic disk storage system is provided for adjusting data transfer parameters, such as channel reference frequency and read/write gate assertion timing, based on measured spin speed variation of data storage media rotated within the system. The system includes a read head selectively positionable adjacent to tracks in the data storage media for sensing when servo position fields pass by the head. In response, the head transmits a servo timing signal to an included controller. The controller receives a first and a second servo timing signal and determines the spin speed of the data storage media. The system further includes memory that stores the nominal or design spin speed for data storage media within the system. The controller is configured, with firmware and/or hardware, to determine spin speed error by comparing the measured spin speed with the nominal spin speed. To compensate for any variation in spin speed, the controller includes firmware and/or hardware to adjust at least one of the data transfer parameters, thereby correcting for spin speed error and increasing the efficiency of data storage.